Aneuploidy is a hallmark of cancer and the leading cause of miscarriages in humans. Determining the causes and consequences of aneuploidy is thus vital for understanding the principles underlying tumor formation and infertility. The long-term goal of our studies is to define the mechanisms that prevent chromosome mis-segregation and thus aneuploidy and the consequences of aneuploidy on cell proliferation at the molecular level. The three Specific Aims of this proposal are aimed at addressing these two questions. In Specific Aim 1, we use molecular and biochemical approaches to determine how the Mitotic Exit Network, a regulatory network essential for promoting exit from mitosis, is regulated. In particular, we will determine, how the GTPase of this pathway is controlled and, using this information, develop in vivo read outs for the activity of the GTPase. In Specific Aim 2 we will investigate how a surveillance mechanism known as the spindle position checkpoint affects MEN activity. Specifically, we will determine how the spindle position checkpoint component Kin4 regulates the MEN and test the hypothesis that division of the yeast cell in a MEN inhibitory zone (the mother cell) and a MEN activating zone (the bud) is the mechanism whereby spindle position is sensed in yeast. In Specific Aim 3 will characterize the consequences of aneuploidy. We have discovered that many yeast strains with one or two extra chromosomes are delayed in G1. We will determine the cause(s) of this G1 delay. Furthermore we will identify genes that mediate this G1 delay by identifying loss of function alleles and high copy plasmids that shorten the G1 delay in aneuploids. Their analysis will shed light on how aneuploidy delays cell cycle progression.